RIght Click in Windows Desktop, Select "Screen Resolutions", under resolution, you can see the different resolutions that you can set. Whatever is recommended is the native resolution of your monitor. Choose it for best results.
Edit: I see that it is a CRT monitor. In this case, there is no native resolution. You can set it up to 1,920 x 1,440 but if you want more fps, you can lower the resolution to any 4:3 Aspect Ratio option.